The present invention relates to a steel member such as tube, plate, bar, wire or the like plated with a Pb-Sn alloy to have an increased corrosion resistance, suitable for use as the material of a fuel tank for gasoline, alcoholic fuel or the like, as well as to a method of producing the same.
Hitherto, steel material plated with Pb-Sn alloy has been used as materials of various parts such as automobile fuel tank, parts of radiator, chemical vessel and so forth, because of superior corrosion resistance, solderability and workability, as well as moderate cost.
However, Pb as the plating metal and the steel can hardly react with each other to make the formation of the alloy layer therebetween difficult. In addition, since the Pb itself is liable to be oxidized, it is difficult to adjust the amount of deposition of the plating alloy, often resulting in a problem of formation of pin holes. It is also to be pointed out that the Pb-Sn alloy which is inherently soft tends to be damaged during handling or the press work to develop scratches or pin holes. This also promote the generation of rust particularly when the material is used under a corrosive environment.
Recently, due to the shortage of crude oil resources, there is an increasing need for alcoholic automobile fuel in which alcohol such as methyl alcohol, ethyl alcohol, methyl tertiary butyl ether or the like (generally referred to as "gasohol" or even pure alcohol fuel. This gives a rise to a demand for steel sheet plated with Pb-Sn alloy having higher corrosion resistance and capable of suppressing the formation of pin holes as compared with conventional steel sheet plated with Pb-Sn alloy.
Various proposals have been made to cope with this demand. For instance, it has been proposed to effect the coarseness of the surface of the substrate material through a heavy pickling for long time and at high temperature, thereby to increase the reactivity and mechanical bonding between the steel and the Pb-Sn plating alloy. It has also been proposed, in order to prevent the generation of pin holes due to inadequate roll squeezing for adjusting the amount of deposition of the plating alloy, to conduct the roll squeezing while covering the entire surface of the plating bath with a flux or palm oil, irrespective of whether the roll squeezing is effected by a pair of rolls to which the flux is deposited or by means of a high-pressure gas wiping.
Also, in the specification of U.S. Pat. No. 3,875,027 (corresponds to Japanese patent publication No. 29986/1976), as well as in Japanese patent publication No. 51426/1980 issued to the present inventors, proposed are new methods effective for preventing generation of pin holes in the Pb-Sn alloy plated steel sheet, in which the steel sheet is beforehand plated with Ni in advance to the plating with molten Pb-Sn alloy.
More specifically, in the method proposed in the specification of U.S. Pat. No. 3,875,027, the plating with Pb-Sn alloy (Terne plating) is effected after a plating with Ni to a thickness of 7.62 to 48.3.mu.. The aforesaid plated steel sheet is used mainly as the pipe material of automobile fuel tanks. In the method disclosed in Japanese patent publication No. 51426/1980, the plating with molten Pb-Sn alloy (Terne plating) is effected after having applied Ni plating to a thickness of 0.03 to 1.0.mu.. According to this method, it is possible to produce steel sheet plated with Pb-Sn alloy having a high corrosion resistance and reduced number of pin holes, even when the pickling before the plating with molten Pb-Sn alloy are simplified.
Another proposed method is to effect an undercoating treatment on the steel surface by forming a single plating layer of Zn, Sn, Cu or the like in advance to the plating with the molten Pb-Sn alloy, as in the case of the pretreatment with Ni stated before. The Zn and Sn in the undercoating layer, however, is liable to be re-dissolved in the plating bath of Pb-Sn alloy during the subsequent plating with Pb-Sn alloy. Also, the Cu does not show a good adhesion to steel so that sufficient effect for preventing the generation of pin holes cannot be obtained. The Ni has an appreciable effect on prevention of the generation of pin holes. However, in the conventional process, the Ni inconveniently forms fragile Ni-Sn layer such as Ni.sub.3 Sn.sub.2, Ni.sub.3 Sn.sub.4 phases or the like to reduce the adhesion between the Ni layer and the Pb-Sn alloy plating layer often resulting in a separation during press work, particularly when the plating dipping time is too long or when the cooling after the plating is effected too gradually.
As is well known, automobile fuel tanks are produced by subjecting the material to a considerably severe shaping or forming process, such as having a shape of 300 mm height including bulging work. It is, therefore, essential that the bonding strength between the steel sheet and the plating layer is sufficiently high to avoid separation of the latter during the forming work.
In general, in order to avoid separation of plating layer in the subsequent mechanical work of metal-plated products, it is necessary that the thickness of the plating layer is not excessively large.
In the case where the metal plating is effected on a steel sheet by employing a heating means as in the case of hot dip plating, heating diffusion or the like, the main plating layer is deposited usually through an intermediate alloy layer or layers. This alloy layer is formed of an intermetallic compound which is generally hard and brittle. For instance, in the case of hot dip plating with molten Zn, an intermetallic compound of Zn-Fe alloy system is formed, while, in the case of ordinary hot dip plating with molten Pb-Sn alloy, an intermetallic compound of Fe-Sn system is formed. It is well known that the bonding strength of the plating layer is seriously decreased if this alloy layer has a large thickness. Thus, a too large thickness of this alloy layer is not preferred also, in order to obtain high bonding strength of the plating layer.
This phenomenon is observed also in the case where the Pb-Sn alloy, which generally is soft and has a high lubricating effect, is used as the coating layer, as well as in the aforementioned case where an undercoating layer of Ni is formed to present the possibility of formation of an alloy of Ni and Sn on which the plating layer of Pb-Sn alloy is formed.
From this point of view, the product plated with Pb-Sn alloy shown in the specification of the U.S. Pat. No. 3,875,027 is mainly directed to steel piping subjected to bending or slight bending, so it cannot be used suitably as the material of automobile fuel tanks, because of a large susceptibility to separation of the plated layer during the mechanical forming work which is conducted under a severe condition.
The Japanese patent publication No. 51426/1980 proposes a technic which offers the following advantages.
(1) Shortening and simplification of pretreatment pickling. PA0 (2) An alloy layer of Ni-Sn alloy is formed uniformly instead of the Fe-Sn alloy to reduce the formation of pin holes after the plating with molten Pb-Sn alloy. PA0 (3) Even when the alloy layer is exposed during the control of the deposition amount, no rust is formed because the alloy layer is of Ni-Sn system, in contrast to the case where the alloy layer is of an Fe-Sn alloy. PA0 (4) The Ni undercoating layer and the Pb-Sn alloy plating layer provide superposed effect on the prevention of generation of pin holes. PA0 (1) The material should have a superior workability and shapability and should exhibit no separation of plating layer during shaping into the form of a fuel tank. PA0 (2) The material should exhibit a high resistance to the corrosive content such as impurities, water content which is formed as a result of a dewing and so forth, not to mention to the gasoline itself. Also, the material should reduce the amount of corrosion products which would cause a clogging in the fuel system. This requirement is particularly strict when an electronic fuel control is adopted. PA0 (3) The material should have longer life than conventional materials, considering that the weight of the fuel tank has to be reduced to cope with the demand for reduction of weight of the automobile as a whole. PA0 (4) The material should exhibit a sufficient corrosion resistance against the alcohol, the water content of which is greater than in gasoline, peroxides (formaldehyde, acetoaldehyde e.t.c.) and organic acids (formic acid, acetic acid e.t.c.) which are formed as a result of oxidation of alcohol. PA0 (1) Steel sheet plated with a Pb-Sn system alloy, having a double layer of an NiSi phase alloy layer formed on the steel surface and a Pb-Sn alloy formed on the Ni-Sn alloy layer, suitable for use as material of automobile fuel tank. PA0 (2) Steel sheet plates with Pb-Sn system alloy, suitable for use as the material of automobile fuel tank, having a tripple layer including an Ni layer, an NiSn alloy layer and a Pb-Sn alloy. PA0 (3) Method of producing a steel sheet plated with Pb-Sn system alloy suitable for use as the material of automobile fuel tank, having the steps of forming on the surface of a steel sheet an Ni layer of a thickness 0.01 to less than 1.mu., dipping the Ni-coated steel sheet in a Pb-Sn system alloy plating bath of held at a temperature of 320.degree. to 400.degree. C. for 1 to 10 seconds and cooling quickly the plated steel sheet down to a temperature below 300.degree. C. within 3 seconds. PA0 (4) Method of producing steel sheet plated with Pb-Sn system alloy suitable for use as the material of automobile fuel tank for alcoholic fuel or pure alcoholic fuel, the method has the steps of plating a steel sheet with Ni, effecting an electroplating with Sn, effecting on electroplating with Pb, and subjecting the plated steel sheet to a heat treatment which is conducted at a temperature ranging between 232.degree. and 400.degree. C. PA0 (1) Namely, according to the invention, among various alloy layers formed as a result of reaction between Ni and Sn, e.g. NiSn, Ni.sub.3 Sn.sub.2, Ni.sub.3 Sn.sub.4, the NiSn which is known as exhibiting the superior corrosion resistance is formed mainly at the interface between the steel surface and the Pb-Sn alloy plating layer or at the interface between the Ni plating layer and the Pb-Sn alloy plating layer. The invention also proposed conditions effective for the formation of the NiSn alloy at such an interface. PA0 (2) The NiSn alloy layer exhibits a good corrosion resistance but has small workability. In order to assure high workability, according to the invention, the thickness of the NiSn layer which adversely affects workability of the plated steel sheet is limited while weighing the corrosion resistance. Namely, the NiSn layer or the sum of Ni layer and NiSn layer, were controlled to have a total thickness of 0.035 to 1.mu.. PA0 (1) In the product hot-dip-plated with Pb-Sn system alloy, the Pb-Sn plating layer is often dropped or stripped partially to form a plating defect through which the alloy layer underlying the Pb-Sn alloy layer is exposed. This plating defect is caused by various reasons such as inadequate control on the amount of metal deposition, non-uniform solidification of the molten Pb-Sn alloy attributable to the surface tension, and so forth. According to the invention, however, the layer exposed through such a plating defect is the NiSn alloy layer which has a superior corrosion resistant and do not product any rust. Namely, the NiSn alloy layer exhibits a much superior corrosion resistance to the Fe-Sn system alloy layer (mainly FeSn.sub.2) formed in the conventional Pb-Sn-plated steel sheet. PA0 (2) For the same reason as the item (1) above, although the final Pb-Sn layer by hot dip plating may have non-uniform thickness to permit an early corrosion of the thin portion thereof, the NiSn layer revealed from such portoin exhibits a sufficiently high corrosion resistance so that the life of the product is prolonged remarkably as compared with the conventional steel sheet hot-dip-plated with Pb-Sn system alloy. PA0 (3) The Pb-Sn system alloy plating layer is liable to be damaged during handling, because it is so soft. However, even if the Pb-Sn system alloy plating layer is damaged, the underlying NiSn layer provides a sufficient corrosion resistance to ensure the longer life of the product plated with Pb-Sn system alloy.
Thanks to the above-listed features, this method permits the production of corrosion-resistant product plated with molten Pb-Sn alloy, even when the pretreatment by pickling is conducted at a high speed and in a simplified manner.
These prior arts explained heretofore are merely the technics for producing products plated with Pb-Sn alloy, highly resistant to corrosion and having reduced number of pin holes. At the present stage, however, it is not considered that a technic has been established for the production of materials having a steel sheet base, a Ni undercoating layer and a plating layer of Pb-Sn alloy formed by hot dip plating, optimum for use as the material of automobile fuel tank, particularly suitable for the material of tank for fuel having alcoholic content.